Aqueous emulsion and process for producing the same

ABSTRACT

A highly stable aqueous emulsion of a polymer predominantly comprised of units of at least one monomer selected from acrylate esters, methacrylate esters, styrenic monomers and diene monomers, which is dispersion-stabilized with a water-soluble compound having a molecular weight of at least 300 and an alcoholic hydroxyl group; characterized by having an average particle diameter of 0.05-5 μm, a calcium chloride chemical stability index of at least 20 and a surface tension of at least 45 mN/m as measured at a solid content of 30 wt. %. The aqueous emulsion is produced by polymerizing a monomer or monomers in the presence of an alcohol and a polymerization initiator generating a peroxide free-radical in an aqueous medium, while the monomer or monomers and the alcoholic hydroxyl group-containing water-soluble compound are incorporated continuously or intermittently into a polymerization system.

TECHNICAL FIELD

This invention relates to an aqueous emulsion of a homopolymer orcopolymer of at least one monomer selected from the group consisting ofan acrylic acid ester monomer, a methacrylic acid ester monomer, astyrenic monomer and a diene monomer, and to a process for producing theaqueous emulsion.

BACKGROUND ART

Emulsions of homopolymers and copolymers prepared by polymerizingvarious ethylenically unsaturated monomers and diene monomers, such as astyrene-butadiene copolymer emulsion, an acrylic acid ester resinemulsion, a methacrylic acid ester resin emulsion and a vinyl esterresin emulsion, are widely used as an adhesive, a coating composition, abinder, a fiber-treating agent and a mortar-ingredient.

The above-mentioned emulsions are produced usually byemulsion-polymerizing an ethylenically unsaturated monomer and/or adiene monomer in the presence of a surface active agent or awater-soluble high-molecular-weight compound such as polyvinyl alcohol(hereinafter abbreviated to “PVA”) or hydroxyethyl cellulose.

By using a surface active agent for polymerization, standing stability,mechanical stability, chemical stability, freeze-thaw stability andpigment-miscibility of the polymer emulsion can be maintained at acertain level but the level is not satisfactorily high. The polymeremulsion produced by an emulsion polymerization procedure using asurface active agent as an emulsifier generally has a low viscosity, andtherefore, when the polymer emulsion is used in a field where a highviscosity is required, a thickening agent must be added or a unsaturatedacid must be copolymerized followed by alkali addition for increase ofviscosity. The addition of a thickening agent and the copolymerizationof an unsaturated acid lead to deterioration of water resistance of afinal product of the emulsion. Further, the surface active agent isliable to migrate in the final product with the result of deteriorationof various properties.

A polymer emulsion prepared by polymerizing vinyl acetate or vinylchloride in the presence of a water-soluble high-molecular-weightcompound such as a PVA polymer as an emulsion stabilizer is satisfactoryin various dispersion stabilities such as mechanical stability andchemical stability and in the fact that a desired viscosity is obtainedby controlling the polymerization procedure and conditions. However,these advantages can be achieved only with limited polymer emulsionsprepared from monomers having a high free-radical polymerizability suchas vinyl acetate and vinyl chloride. Stable polymer emulsions cannot beprepared from monomers having a relatively low free-radicalpolymerizability such as a styrenic monomer, a diene monomer, an acrylicacid ester monomer and a methacrylic acid ester monomer.

A proposal of conducting a polymerization of a styrenic monomer, a dienemonomer, an acrylic acid ester monomer or a methacrylic acid estermonomer by using a PVA polymer having a mercapto group as an emulsionstabilizer was made in Japanese Unexamined Patent Publication(hereinafter abbreviated to “JP-A”) No. S60-197229. But, research of thepresent inventors has revealed that good polymerization stability can beobtained only when a special initiator, such as potassium bromate,capable of generating a free-radical due to the reaction with a mercaptogroup in the PVA polymer is used. Good polymerization stability cannotbe obtained with ordinary initiators. Further, it is pointed out thatthe use of the PVA polymer having a mercapto group has a problem suchthat polymerization inevitably stops when the whole amount of mercaptogroup is consumed.

A process for producing an aqueous polymer emulsion wherein anethylenically unsaturated monomer or a diene monomer is polymerized byusing PVA as a dispersing agent in the presence of a small amount of achain transfer agent having a mercapto group is described in JP-AH8-325312. This process has a problem such that gelation occurs duringpolymerization due to the presence of the chain transfer agent (seeComparative Example 6 given below). A process for producing an aqueousemulsion of a homopolymer or copolymer from an acrylic acid estermonomer, a methacrylic acid ester monomer, a styrenic monomer, a dienemonomer or a halogenated vinyl monomer is described in JP-A H8-104703.In this process, a special vinyl alcohol polymer must be used as adispersing agent, which satisfies the special requirements for theviscosity average degree of polymerization, the ratio of weight averagemolecular weight/number average molecular weight, and the maximum peakof molecular weight in the molecular weight distribution, and therefore,this process is limited in utilization.

Thus, a process for stably producing an aqueous emulsion of ahomopolymer or copolymer from an acrylic acid ester monomer, amethacrylic acid ester monomer, a styrenic monomer, a diene monomer or ahalogenated vinyl monomer has not proposed, wherein these monomers arepolymerized by using a protective colloid of a general water-solublehigh-molecular-weight compound as a dispersing agent.

DISCLOSURE OF THE INVENTION

A primary object of the present invention is to provide an aqueouspolymer emulsion having an extremely enhanced stability, and a processfor producing the aqueous polymer emulsion by homopolymerizing orcopolymerizing an acrylic acid ester monomer, a methacrylic acid estermonomer, a styrenic monomer or a diene monomer by using as a dispersingagent a water-soluble high molecular-weight compound having an alcoholichydroxyl group which is a protective colloid of a general water-solublehigh-molecular-weight compound.

Through a research into a process for stably producing an aqueouspolymer emulsion from an acrylic acid ester monomer, a methacrylic acidester monomer, a styrenic monomer or a diene monomer by using as adispersing agent a protective colloid of a general water-solublehigh-molecular-weight compound, the present inventors have found thatthe above-mentioned object can be achieved by a process wherein thepolymerization is carried out by using a vinyl alcohol polymer as adispersing agent, while the monomer or monomers are incorporated into apolymerization system in a special manner. Based on this finding, thepresent invention has been completed.

Thus, in one aspect of the present invention, there is provided anaqueous emulsion of a homopolymer consisting of units of a monomerselected from the group consisting of an acrylic acid ester monomer, amethacrylic acid ester monomer, a styrenic monomer and a diene monomer,or a copolymer predominantly comprised of units of at least one monomerselected from said group of monomers, which is dispersion-stabilizedwith a water-soluble high-molecular-weight compound having a molecularweight of at least 300 and having an alcoholic hydroxyl group;

characterized in that said homopolyme or copolymer is in the form ofparticles having an average particle diameter of 0.05 μm to 5 μm andsaid aqueous emulsion has a calcium chloride chemical stability index ofat least 20 and a surface tension of at least 45 mN/m as measured at asolid content concentration of 30% by weight;

that the amount of the water-soluble high-molecular-weight compoundhaving an alcoholic hydroxy group is in the range of 0.01 to 100 partsby weight based on 100 parts by weight of the homopolymer or thecopolymer;

that at least part of the water-soluble high molecular-weight compoundhaving an alcoholic hydroxyl group is graft-bonded to the homopolymer orcopolymer; and further

that the amount of the homopolymer or copolymer having graft-bondedthereto the water-soluble high-molecular-weight compound having analcoholic hydroxyl group is in the range of 0.5% to 30% by weight basedon the sum of the homopolymer or copolymer having grafted thereto thewater-soluble high-molecular-weight compound having an alcoholichydroxyl group and the homopolymer or copolymer, to which thewater-soluble high-molecular-weight compound having an alcoholichydroxyl group has not been grafted.

In another aspect of the present invention, there is provided a processfor producing the above-mentioned aqueous emulsion, characterized inthat either one kind of a monomer selected from the group consisting ofan acrylic acid ester monomer, a methacrylic acid ester monomer, astyrenic monomer and a diene monomer, or a monomer mixture predominantlycomprised of at least one kind of a monomer selected from said group ofmonomers, is polymerized in an aqueous medium in the presence of analcohol and a polymerization initiator generating a peroxidefree-radical as the sole polymerization initiator, while the monomer ormonomer mixture and a water-soluble high-molecular-weight compoundhaving a molecular weight of at least 300 and having an alcoholichydroxyl group are incorporated continuously or intermittently into apolymerization system.

BEST MODE FOR CARRYING OUT THE INVENTION

The monomer used for the preparation of a polymer in the aqueousemulsion of the invention is a monomer selected from the groupconsisting of an acrylic acid ester monomer, a methacrylic acid estermonomer, a styrenic monomer and a diene monomer, or a monomer mixturepredominantly comprised of at least one kind of a monomer selected fromsaid group of monomers.

As specific examples of the acrylic acid ester monomer, there can bementioned methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexylacrylate, dodecyl acrylate, and dimethylaminoethyl acrylate and itsquarternized product.

As specific examples of the methacrylic acid ester monomer, there can bementioned methyl methacrylate, ethyl methacrylate, butyl methacrylate,2-ethylhexyl methacrylate, dodecyl methacrylate, dimethylaminoethylmethacrylate and its quarternized product, and ethylene glycoldimethacrylate.

As specific examples of the styrenic monomer, there can be mentionedstyrene, α-methylstyrene, vinyltoluene, and p-styrenesulfonic acid and asodium salt or potassium salt thereof.

As specific examples of the diene monomer, there can be mentioned1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene,2-chloro-1,3-butadiene and 1,3-pentadiene.

Monomers, which are capable of being copolymerized with theabove-mentioned monomers used as the predominant ingredient for thecopolymerization, include for example halogenated monomers andcarboxylic acid vinyl ester monomers. These copolymerizable monomers areused in an amount such that the object and effects of the invention areattained. The amount thereof is usually not larger than 50% by weightbased on the total weight of the monomers.

As specific examples of the halogenated vinyl monomer, there can bementioned vinyl chloride, vinylidene chloride, vinyl fluoride andvinylidene fluoride. As specific examples of the carboxylic acid vinylester monomer, there can be mentioned vinyl formate, vinyl acetate,vinyl propionate, vinyl versatate, vinyl pivalate and isopropenylacetate. As specific examples of the halogenated vinyl monomer, therecan be further mentioned unsaturated monomcarboxylic acids such asacrylic acid and methacrylic acid; polycarboxylic acids and partialesters thereof such as fumaric acid, maleic acid, itaconic acid, maleicanhydride, phthalic anhydride, trimellitic anhydride and itaconicanhydride; olefins such as ethylene, propylene, 1-butene and isobutene;vinyl ethers such as methyl vinyl ether, n-propyl vinyl ether, i-propylvinyl ether, n-butyl vinyl ether, i-butyl vinyl-ether, t-butyl vinylether and dodecyl vinyl ether; nitriles such as acrylonitrile andmethacrylonitrile; allyl compounds and methallyl compounds such as allylacetate, methallyl acetate, allyl chloride and methallyl chloride, saltsand esters of unsaturated carboxylic acids such as dimethyl fumarate,diethyl maleate and diisopropyl itaconate; vinylsilyl compounds such asvinyltrimethoxysilane; and vinylpridine and N-vinylpyrrolidone.

In the process for producing the aqueous polymer emulsion of theinvention, first, the above-mentioned monomer or monomers and awater-soluble high-molecular-weight compound having a molecular weightof at least 300 and having an alcoholic hydroxyl group (hereinafterabbreviated to “alcoholic hydroxyl group-containinghigh-molecular-weight compound”) are incorporated in a polymerizationsystem where polymerization is conducted in the presence of an alcoholand a polymerization initiator capable of being decomposed to generate aperoxide free-radical as the sole polymerization initiator in an aqueousmedium. By the term “aqueous medium” used herein we mean water and amixture of water and an optional water-soluble solvent such as analcohol. The monomer or monomers and the alcoholic hydroxylgroup-containing high-molecular-weight compound may be incorporated inthe polymerization system either separately or as a mixture in anemulsion form comprising the monomer or monomers, the alcoholic hydroxylgroup-containing high-molecular-weight compound and water. In the casewhen the monomer or monomers and the alcoholic hydroxyl group-containinghigh-molecular-weight compound are separately incorporated, these twoingredients are preferably simultaneously incorporated. If a largeamount of the monomer or monomers is incorporated prior to the alcoholichydroxyl group-containing high-molecular-weight compound, coagulatedmass is liable to occur. In contrast, if a large amount of the alcoholichydroxyl group-containing high-molecular-weight compound is incorporatedprior to the monomer or monomers, viscosity of the polymerization systemincreases and coagulated mass is liable to occur. It is not essentialbut preferable that the incorporation of the two ingredients iscompleted at the same time. The monomer or monomers and the alcoholichydroxyl group-containing high-molecular-weight compound areincorporated continuously or intermittently. It is essential toincorporate the monomer or monomers and the alcoholic hydroxylgroup-containing high-molecular-weight compound with the advance ofpolymerization. If the homopolymer or copolymer is produced by apolymerization procedure other than an emulsion polymerizationprocedure, and thereafter the homopolymer or copolymer or an aqueoussolution thereof is emulsified in a forced emulsification manner or aself-emulsification manner by adding the alcoholic hydroxylgroup-containing high-molecular-weight compound, then the resultingaqueous polymer emulsion is greatly inferior in various stabilities tothe aqueous polymer emulsion of the present invention. Also, if anaqueous emulsion of the homopolymer or copolymer is produced by anemulsion polymerization procedure using a surface active agent or a postemulsification procedure, followed by incorporation of the alcoholichydroxyl group-containing high-molecular-weight compound, then theresulting aqueous polymer emulsion is similarly inferior in variousstabilities to the aqueous polymer emulsion of the present invention.

If the whole amount of the monomer or monomers used is incorporatedalone or along with the alcoholic hydroxyl group-containinghigh-molecular-weight compound in an initial stage of polymerization,followed by incorporation of a polymerization initiator to initiate thepolymerization, then the conversion of monomer becomes unstable andother problems arise and the object of the present invention cannot beattained.

In the process for producing the aqueous polymer emulsion of the presentinvention, it is also essential to carry out the polymerization in thepresence of an alcohol for obtaining an aqueous polymer emulsionsuperior in various stabilities. The alcohol may be incorporated in apolymerization system as the solvent or a part thereof for dissolvingthe polymerization initiator therein. The alcohol used is notparticularly limited, and may be either monohydric or polyhydric. Asspecific examples of the alcohol, there can be mentioned methanol,ethanol, propanol, butanol, ethylene glycol, propylene glycol and.glycerol. The amount of the alcohol is usually in the range of 0.5 to 50parts by weight, preferably 1 to 30 parts by weight and more preferably2 to 20 parts by weight, based on 100 parts by weight of the monomer ormonomer mixture.

If an alcohol is not present or is present only to an extremely minoramount, an aqueous polymer emulsion having an enhanced stability cannotbe obtained. The reason for which the stability of the aqueous polymeremulsion is enhanced by the presence of an alcohol is not clearlyelucidated, but it is presumed to be based on the following mechanismalthough the present invention is not bound thereto. If an alcohol isnot present, hydrogen abstraction from PVA polymer by a peroxidefree-radical easily occurs whereby PVA radicals are produced and thesePVA radicals are bound together to give a precipitate. In contrast, whenan alcohol is present, a peroxide free-radical reacts with the alcoholto effect hydrogen abstraction from the alcohol to produce an alcoholfree-radical. The thus-produced alcohol free radical exhibits a lowerrate of hydrogen abstraction from PVA polymer than the hydrogenabstraction rate of the peroxide free-radical from PVA polymer.Therefore, the production of PVA radicals is suppressed and thus bindingof PVA radicals is minimized with the result of avoidance ofunstablization of the polymerization system.

In the above-mentioned polymerization process, it is also essential touse a polymerization initiator generating a peroxide free-radical as thesole polymerization initiator. By the term “peroxide free-radical” usedherein we mean a free-radical having a structure such that is producedby scission of O—O bond of a peroxide. As specific examples of thepolymerization initiator generating a peroxide free-radical, there canbe mentioned water soluble peroxides such as potassium persulfate,ammonium persulfate and hydrogen peroxide; oil soluble peroxides such astert.-butyl hydroperoxide, benzoyl peroxide and di-tert.-butyl peroxide;and redox initiators such as those which are combinations of, forexample, a peroxide with sodium hydrogen bisulfite or other reducingagents. Of these, water soluble peroxides are preferable. A persulfatesalt is most preferable. The amount of the polymerization initiator isusually in the range of 0.05 to 3 parts by weight, preferably 0.1 to 2parts by weight based on 100 parts by weight of the monomer or monomers.If a polymerization initiator incapable of generating a peroxidefree-radical is used, an aqueous emulsion having good stability cannotbe used.

The procedure by which the polymerization initiator is incorporated isnot particularly limited. As examples of the procedure, there can bementioned a procedure of incorporating the entire amount of thepolymerization initiator in a polymerization vessel before thepolymerization; a procedure of incorporating a part of thepolymerization initiator in a polymerization vessel before thepolymerization, and, after the commencement of polymerization,additionally incorporating the remainder of polymerization initiator ata predetermined time; and a procedure of incorporating a part of thepolymerization initiator in a polymerization vessel before thepolymerization, and, after the commencement of polymerization,continuously or intermittently incorporating the remainder ofpolymerization initiator along with the addition of the monomer ormonomers and the alcoholic hydroxyl group-containing water-solublehigh-molecular-weight compound.

The alcoholic hydroxyl group-containing water-solublehigh-molecular-weight compound used in the present invention includes,for example, vinyl alcohol polymers such as polyvinyl alcohol andmodification products thereof; a saponification product of a copolymerof vinyl acetate with acrylic acid, methacrylic acid or maleicanhydride; cellulose derivatives such as alkyl cellulose, hydroxyalkylcellulose, alkylhydroxyalkyl cellulose and carboxymethyl cellulose;starch derivatives such as alkyl starch, carboxymethyl starch andoxidized starch; gum arabic, tragacanth gum; and polyalkylene glycol. Ofthese, vinyl alcohol polymers are preferable in view of commerciallyavailable polymers with stable quality.

The vinyl alcohol polymers are not particularly limited provided thatthey are substantially water-soluble and capable of producing a stableemulsion. Usually the vinyl alcohol polymers are obtained by saponifyingby an ordinary procedure vinyl ester polymers which are prepared bypolymerizing vinyl monomer or monomers consisting of or predominantlycomprised of a vinyl ester monomer by a convention polymerizationprocedure, and which include a homopolymer of a vinyl ester monomer, acopolymer of at least two vinyl ester monomers, and a copolymer of avinyl ester monomer with other ethylenically unsaturated monomer. Thevinyl alcohol polymers may be modified, for example, by an epoxy groupintroduced into the main chain, side chains or terminals of the polymer.

Any vinyl ester monomers can be used provided that they are capable ofbeing subjected to free-radical polymerization. As specific examples ofthe vinyl ester monomer, there can be mentioned vinyl formate, vinylacetate, vinyl propionate, isopropenyl acetate, vinyl valerate, vinylcaprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl versatateand vinyl pivalate. Of these, vinyl acetate is most popularly usedbecause it is commercially available and inexpensive.

If desired, a monomer copolymerizable with the vinyl ester monomer canbe used for producing a copolymer of the vinyl ester monomer. Asexamples of the copolymerizable monomer, there can be mentioned olefinssuch as ethylene, propylene, 1-butene and isobutene, unsaturatedcarboxylic acids and anhydrides thereof such as acrylic acid,methacrylic acid, fumaric acid, maleic acid, itaconic acid, maleicanhydride, phthalic anhydiride, trimellitic anhydride and itaconicanhydride; acrylic acid esters such as methyl acrylate, ethyl acrylate,n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, t-butylacrylate, 2-ethylhexyl acrylate, dodecyl acrylate and octadecylacrylate; methacrylic acid esters such as methyl methacrylate, ethylmethacrylate, n-propyl methacrylate, i-propyl methacrylate, n-butylmethacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecylmethacrylate and octadecyl methacrylate; unsaturated carboxylic acidesters such as dimethyl fumarate, diethyl maleate and diisopropylitaconate; vinyl ethers such as methyl vinyl ether, n-propyl vinylether, i-propylvinyl ether, n-butyl vinyl ether, i-butyl vinyl ether,t-butyl vinyl ether, dodecyl vinyl ether and stearyl vinyl ether;nitriles such as acrylonitrile and methacrylonitrile; vinyl orvinylidene halides such as vinyl chloride, vinylidene chloride, vinylfluoride and vinylidene fluoride; allyl compounds allyl acetate andallyl chloride; sulfonic acid group-containing compounds such asethylenesulfonic acid, allylsulfonic acid, methallylsulfonic acid and2-acrylamide-2-methylpropanesulfonic acid; vinylsilane compounds such asvinyltrimethoxysilane; and quaternary ammonium group-having monomerssuch as 3-acrylamidepropyltrimethylammonium chloride and3-methacrylamidepropyltrimethylammonium chloride.

The saponification degree of the vinyl ester polymer varies dependingupon the presence or absence of a modification group and the kind of themodification group. Preferably the saponification degree is in the rangeof 40 to 99.99% by mole, more preferably 50 to 99.9% by mole and mostpreferably 60 to 99.9% by mole in view of the water-solubility and otherproperties of the resulting vinyl alcohol polymer. If the saponificationdegree of the vinyl ester polymer is smaller than 40% by mole,dispersion stability of polymer particles is reduced.

The viscosity average degree of polymerization of the vinyl esterpolymer is usually in the range of 50 to 8,000, preferably 100 to 6,000and more preferably 100 to 5,000. If the viscosity average degree ofpolymerization is smaller than 50, the polymerization stability is poor.In contrast, viscosity average degree of polymerization is larger than8,000, the viscosity of the aqueous polymer emulsion becomes very highand the removal of heat at the step of production of the emulsionbecomes difficult.

The amount of the alcoholic hydroxyl group-containing water-solublehigh-molecular-weight compound is in the range of 0.01 to 100 part byweight, preferably 0.05 to 50 parts by weight and more preferably 1 to20 parts by weight based on 100 parts by weight of the monomer ormonomers. When the amount of this compound is smaller than 0.01 part byweight, the stability at the step of polymerization is poor and asalient amount of coagulated mass are produced, the mechanical stabilityand chemical stability of the aqueous polymer emulsion are reduced, andthe strength of film obtained therefrom is reduced. In contrast, whenthe amount of this compound is larger than 100 parts by weight, removalof the heat of reaction becomes difficult due to the viscosity increaseof the polymerization system, the resulting polymer emulsion becomes toothick in viscosity and becomes difficult to handle, and the waterresistance of film obtained therefrom is reduced.

In the emulsion polymerization, conventional emulsifiers includingnonionic, anionic, cationic and amphoteric surface active agents can beused provided that the object and effect of the invention are attained.Of these anionic surface active agents are preferable.

As specific examples of the emulsifiers, there can be mentioned anionicsurface active agents such as higher alcohol sulfate esters,alkylbenzenesulfonate salts, aliphatic sulfonate salts, polyoxyethylenealkylarylsufonate salts and polyphosphate salts; nonionic surface activeagents such as polyethylene glycol alkyl esters, polyethylene glycolalkylphenyl ethers and polyethylene glycol alkyl ethers; cationicsurface active agents such as aliphatic amine salts and quaternaryammonium salts thereof, aromatic quaternary ammonium salts andheterocyclic quaternary ammonium salts; and amphoteric surface activeagents such as carboxybetaine, sulfobetaine, aminocarboxylate salts andimidazoline derivatives. These emulsifiers may be used either alone oras a combination of at least two thereof. In the case when theemulsifier is used, the amount thereof is usually in the range of 0.1 to5 parts by weight based on 100 parts by weight of the monomer ormonomers. When the amount of the emulsifier is too large, the alcoholdispersibility become poor.

The relative amount of the monomer or monomers used for polymerizationto the aqueous polymerization medium is not particularly limited but isusually in the range of 10 to 70 parts by weight, preferably 20 to 50parts by weight based on 100 parts by weight of the monomer or monomers.

The rate of addition of the monomer or monomers is not particularlylimited, but the monomer or monomers are incorporated preferably in amanner such that the conversion is maintained at 10% by weight or moreduring polymerization. Preferably the conversion is maintained at 20% byweight or more and more preferably at 30% by weight or more duringpolymerization. If the rate of addition of the monomer or monomers istoo high, the conversion becomes undesirably low and coarse particlesare liable to occur. In contrast, if the rate of monomer addition is toolow, the viscosity of the polymerization system tends to increase.

Therefore, the time required for addition of the monomer or monomers andthe alcoholic hydroxyl group-containing high-molecular-weight compoundis usually at least one hour, preferably at least 2 hours and not morethan 20 hours.

The polymerization temperature is not particularly limited, but isusually in the range of 0° C. to 100° C., preferably 5° C. to 95° C.

After completion of the addition of the monomer or monomers, if desired,the polymerization is continued until a desired polymerizationconversion is reached, and then the polymerization is terminated. Thetermination of polymerization can be effected by adding a polymerizationstopper or cooling the polymerization system. Other polymerizationconditions and procedures are not particularly limited, and those whichare hitherto known in the conventional emulsion polymerizationprocedures can be employed.

A chain transfer agent can be used in the polymerization. An effect ofenhancing the polymerization stability can be obtained by using a chaintransfer agent.

The chain transfer agent used is not particularly limited provided thata chain transfer occur. As specific examples of the chain transferagent, there can be mentioned mercaptans such as n-hexyl mercaptan,n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecylmercaptan and n-stearyl mercaptan; mercapto group-containing compoundssuch as thioglycollic acid, thiomalic acid and 2-ethylhexylthioglycollate; xanthogen compounds such as dimethylxanthogen disulfideand diisopropylxanthogen disulfide; α-methylstyrene dimers such asα-methylstyrene dimer, 2,4-diphenyl-4-methyl-1-pentene,2,4-diphenyl-4-methyl-2-pentene and 1,1,3-trimethyl-3-phenylindane;thiuram compounds such as terpinolene, tetramethylthiuram disulfide,tetraethylthiuram disulfide and tetramethylthiuram monosulfide; phenoliccompounds such as 2,6-di-t-butyl-4-methylphenol and styrenated phenol;allyl compounds such as allyl alcohol, acrolein and methacrolein;halogenated hydrocarbon compounds such as dichloromethane,dibromomethane, carbon tetrachloride and carbon tetrabromide; vinylethers such as α-benzyloxystyrene, α-benzyloxyacrylonitrile andα-benzyloxyacrylamide; and triphenylethane and pentaphenylethane.

Of these, mercapto group-containing compounds are preferable in view ofthe efficiency of chain transfer. As the mercaptp group-containingcompounds, preferable are those which have not more than 50 carbonatoms, more preferably not more than 30 carbon atoms and especiallypreferably not more than 20 carbon atoms. As examples of especiallypreferable mercapto group-containing compounds, there can be mentionedalkyl mercaptans such as n-octyl mercaptan, n-dodecyl mercaptan andt-dodecyl mercaptan, and 2-mercaptoethanol and 3-mercaptopropionic acid.

When a chain transfer agent is used, the amount thereof is usually inthe range of 0.01 to 5 parts by weight based on 100 parts by weight ofthe monomer or monomers. If the amount of the chain transfer agent issmaller than 0.01 part by weight, the effect of enhancing polymerizationstability is minor and the practicability is poor. In contrast, if theamount thereof is larger than 5 parts by weight, the polymerizationstability is poor, and the resulting polymer has a very low molecularweight with the result of deterioration in physical properties of thepolymer emulsion.

The procedure of incorporating a chain transfer agent is notparticularly limited, and it may incorporated at a time, orintermittently or continuously.

Auxiliaries such as a plasticizer and an antifoaming agent can be addedat the step of or after polymerization.

By the process of the present invention, an aqueous polymer emulsionhaving the following characteristics can be obtained.

The aqueous polymer emulsion of the invention has an average particlediameter of 0.05 to 5 μm, preferably 0.2 to 2 μm.

The mechanical stability of the aqueous polymer emulsion of theinvention as measured by the Marlon type mechanical stability test at apH value of 8 is excellent, i.e., the amount of coagulated mass producedis not larger than 0.1% by weight.

There is no need of introducing a carboxylic group in the polymermolecule, and therefore, the change of viscosity of the polymer emulsiondepending upon the pH value is not observed, and film formed from thepolymer emulsion exhibits enhanced water-resistance.

The aqueous polymer emulsion of the invention has excellent chemicalstability, especially for a divalent inorganic salt. For example, evenwhen an aqueous calcium chloride solution having a concentration of 30%by weight is incorporated with the same amount of the aqueous polymeremulsion having a concentration of 30% by weight, coagulation does notoccur. The chemical stability index for calcium chloride of the aqueouspolymer emulsion of the invention is at least 20, preferably at least30. The definition of the chemical stability index for calcium chloridewill be explained below.

The aqueous polymer emulsion of the invention does not contain a surfaceactive agent, and therefore, has a surface tension of at least 45 mN/m,preferably at least 50 mN/m.

The aqueous polymer emulsion of the invention does not contain a surfaceactive agent, and therefore, it is considered that the emulsion exhibitsenhanced water-resistance and superior performances as adhesives.

The aqueous emulsion of the invention is an emulsion of a homopolymerconsisting of units of a monomer selected from the group consisting ofan acrylic acid ester monomer, a methacrylic acid ester monomer, astyrenic monomer and a diene monomer, or a copolymer predominantlycomprised of units of at least one monomer selected from said group ofmonomers, and at least part of the water-soluble high-molecular-weightcompound having an alcoholic hydroxyl group added as a dispersionstabilizer at the step of polymerization is graft-bonded to thehomopolymer or copolymer. It is presumed that the excellent stability ofthe aqueous polymer emulsion of the invention is due to the graft-bondedwater-soluble high-molecular-weight compound having an alcoholichydroxyl group.

The amount of said part of the homopolymer or copolymer havinggraft-bonded thereto the water-soluble high-molecular weight compoundhaving an alcoholic hydroxyl group is usually at least 0.5% by weightbased on the sum of said part of the homopolymer or copolymer havinggrafted thereon the water-soluble high-molecular-weight compound havingan alcoholic hydroxyl group and the homopolymer or copolymer, onto whichthe water-soluble high-molecular-weight compound having an alcoholichydroxyl group has not been grafted. If the amount of the said part ofthe homopolymer or copolymer having graft-bonded thereto the alcoholichydroxyl group-containing high-molecular-weight compound is smaller than0.5% by weight, the aqueous emulsion is poor in stability. There is noupper limit of the relative amount of the homopolymer or copolymerhaving graft-bonded thereto the alcoholic hydroxyl group-containingwater-soluble high-molecular-weight compound, but said amount is usuallynot larger than 30% by weight.

A free alcoholic hydroxyl group-containing water-solublehigh-molecular-weight compound and a homopolymer or copolymer havinggraft-bonded thereto the alcoholic hydroxyl group-containingwater-soluble high-molecular-weight compound, which are present in theaqueous emulsion, can be separated from each other, for example, bycentrifugal separation. That is, the free alcoholic hydroxylgroup-containing water-soluble high-molecular-weight compound remains inthe aqueous emulsion, and the homopolymer or copolymer havinggraft-bonded thereto the alcoholic hydroxyl group-containingwater-soluble high-molecular-weight compound is precipitated.

Further, the homopolymer or copolymer having graft-bonded thereto thealcoholic hydroxyl group-containing water-soluble high-molecular-weightcompound can be separated from the homopolymer or copolymer, onto whichthe alcoholic hydroxyl group-containing water-solublehigh-molecular-weight compound has not been grafted, by treating thesewith a peroxide free-radical whereby the alcoholic hydroxylgroup-containing water-soluble high-molecular-weight compound isinsolubilized. The amount of the alcoholic hydroxyl group-containingwater-soluble high-molecular-weight compound in the homopolymer orcopolymer having graft-bonded thereto the water-solublehigh-molecular-weight compound can be determined, for example, by aprocedure wherein the hydroxyl group in the compound are converted to anappropriate derivative and the derivative is analyzed by NMR or othermeans.

The invention will now be specifically described by the followingworking examples that by no means limit the scope of the invention. %and parts in the working examples are by weight unless otherwisespecified, and the amount of a latex is expressed in terms of the weightof the solid content.

The experimental data given in the working examples were determined bythe following methods.

(1) Polymerization Stability (%)

An aqueous polymer emulsion as obtained by polymerization is filteredthrough a wire mesh with 200 mesh size, and the coagulated massremaining on the filter is dried in a dryer maintained at a temperatureof 105° C. and the dried mass is weighed. The polymerization stabilityis expressed in terms of the weight % of the dried coagulated mass basedon the weight of the total solid content in the aqueous polymeremulsion.

(2) Weight Average Particle Diameter (μm)

The weight average particle diameter is determined by a granulometerColter LS230 (supplied by Colter Co.).

(3) Particle Diameter Distribution

The particle diameter distribution is determined determined by agranulometer Colter LS230 (supplied by Colter Co.).

(4) Viscosity (mP.s)

The viscosity is measured by a BM viscometer using #4 rotor at 20° C.and 60 rpm.

(5) Surface Tension (mN/m)

The surface tension is determined on an aqueous polymer emulsion havinga solid content of 30% by weight by using an auto-tension meter (Type6801ES supplied by Rigousha K.K.) at 20° C.

(6) Chemical Stability (Calcium Chloride Chemical Stability Index)

An aqueous polymer emulsion is filtered through a wire mesh with 100mesh size, and the solid content in the emulsion is adjusted to 30%. Anequal volume of an aqueous calcium chloride solution containing 30% ofcalcium chloride is incorporated with the aqueous polymer solutionhaving a solid content of 30% by weight. The mixture is filtered througha wire mesh with 100 mesh size and it is observed whether coagulatedmass remains or not on the wire mesh. This test is repeated by usingaqueous calcium chloride solutions having different calcium chlorideconcentrations. The calcium chloride chemical stability index isexpressed in terms of the highest concentration (×%) of calcium chloridein an aqueous calcium chloride solution, at which concentration,coagulated mass does not remain on the wire mesh, when a mixture of thecalcium chloride solution with the same amount of an aqueous polymersolution having the same solid content is filtered through the wiremesh. As the numerical value of the calcium chloride chemical stabilityindex is larger, the aqueous polymer emulsion has a better chemicalstability.

It is to be noted that chemical stability index can also be determinedby using other electrolyte, e.g., sodium chloride, instead of calciumchloride.

(7) Mechanical Stability (CG8) (%)

Ph value of an aqueous polymer emulsion is adjusted to 8±0.1, and theaqueous emulsion is filtered through a wire mesh with 100 mesh size.Then the solid content in the emulsion is adjusted to 30%. The emulsionis filtered through a wire mesh with 100 mesh size and then subjected toa Marlon mechanical stability test at a revolution of 1,000 rpm and aload of 15 kg for 10 minutes. The tested emulsion is filtered through awire mesh with a 100 mesh size, and the coagulated mass remaining on thewire mesh is dried and weighed. The mechanical stability is expressed interms of the weight % of the dried coagulated mass (CG 8) based on theweight of the total solid content in the aqueous polymer emulsion.

(8) Graft Percentage (%)

The solid content in an aqueous polymer emulsion as obtained bypolymerization is adjusted to 10% by weight, and 60 g of the emulsion isused as a test specimen. The specimen is subjected to centrifugalseparation at a temperature of 5° C. and a revolution of 13,000 rpm for60 minutes and a supernatant liquid is recovered in an amount of 40 g.40 g of distilled water is added to the sediment layer (40 g) andstirred uniformly. Then the liquid is again subjected to centrifugalseparation under the same conditions as those in the first centrifugalseparation, and a supernatant liquid is recovered in an amount of 40 g.The sediment layer is processed in the same manner as mentioned above.The supernatant liquids obtained by the centrifugal separation repeatedthree times are collected together to obtain 120 g of liquid. Thecontent of a solid in 120 g of the liquid, that is the amount (A) of thealcoholic hydroxyl group-containing water-soluble high-molecular-weightcompound which has not been grafted onto the homopolymer or copolymer.

The amount (C) of the alcoholic hydroxyl group-containing water-solublehigh-molecular-weight compound which has been grafted onto thehomopolymer or copolymer is calculated by deducting the amount (A) fromthe total amount (B) of the alcoholic hydroxyl group-containingwater-soluble high-molecular-weight compound in the specimen, i.e., thesum of the alcoholic hydroxyl group-containing water-solublehigh-molecular-weight compound having been grafted onto the homopolymeror copolymer, plus the alcoholic hydroxyl group-containing water-solublehigh-molecular-weight compound not having been grafted onto thehomopolymer or copolymer.

The graft percentage, i.e., the ratio in percentage of the amount (C) ofthe hydroxyl group-containing water-soluble high-molecular-weightcompound having grafted onto the homopolymer or copolymer to the amountof the homopolymer or copolymer is expressed by the following formula:

 graft percentage=[C/(6−B)]×100(%)

EXAMPLE 1

To 90 parts of deionized water, a monomer mixture of 80 parts of styreneand 20 parts of ethyl acrylate, and 5 parts of polyvinyl alcohol(PVA-205, supplied by Kuraray Co., degree of polymerization: 550, degreeof saponification: 88.5 mol %) were added, and the resulting mixture wasstirred to give an emulsion of monomers.

A glass reaction vessel equipped with a reflux condenser, a droppingfunnel, a thermometer, a nitrogen gas-blowing inlet and a stirrer wascharged with 57 parts of distilled water and 3 parts of ethanol, and themixture was heated to 80° C. Then, while the temperature was maintainedat 80° C., a solution of 0.5 g of ammonium persulfate (polymerizationinitiator) in 10 parts of deionized water was added to the content inthe reaction vessel. Two minutes after the addition of thepolymerization initiator, addition of the above-mentioned monomeremulsion into the reaction vessel was commenced. The addition of themonomer emulsion was continued over a period of 4 hours, and, after thecompletion of addition, stirring was further continued for two hours.Then the conversion was determined, and the polymerization mixture wascooled to give a polymer emulsion A. The preparation conditions employedfor polymer emulsion A are summarized in Table 1.

The solid content in polymer emulsion A was adjusted to 35%. Theadjusted polymer emulsion A had a pH value of 2.2, a B-type viscosity of100 Pa.s and a surface tension of 56 mN/m (see Table 3).

The chemical stability of polymer emulsion A was examined. A coagulatedmass was not found even in an aqueous 30% calcium chloride solution,namely, the calcium chloride chemical stability index was at least 30.The Marlon mechanical stability test on polymer emulsion A having a pHvalue of 8 revealed that its CG8 was 0.0015%.

The solid content in polymer emulsion A was adjusted to 10%, and thegraft percentage was determined on 60 g of the emulsion A having a solidcontent of 10%. A centrifugal separator H-2000A supplied by a domesticcentrifugal separator maker was used. The graft percentage wascalculated as 3.2% from the total solid concentration in the collectedsupernatant liquids of 0.09%, the total solid content (A) in thecollected supernatant liquids of 0.11 g and the total amount (B) of thewater-soluble high-molecular-weight protective colloid of 0.29 g by thefollowing formula:

[0.18/(6−0.29)×100=3.2%

The ratio in % of the PVA having grafted onto the copolymer to the totalamount of PVA used for the test was 62% as calculated by the followingformula:

(0.18/0.29)×100=62%

EXAMPLES 2-5

Using the monomer compositions and the alcoholic hydroxylgroup-containing water-soluble high-molecular-weight compound, which areshown in Table 1, aqueous polymer emulsions B, C and D were prepared andthe characteristics of polymer emulsions B, C and D were evaluated bythe same procedures as described in Example 1. PVA-202E was polyvinylalcohol having a degree of polymerization of 2,050 and a degree ofsaponification of 88%, supplied by Kuraray Co. The test results areshown in Table 1 and Table 3.

EXAMPLE 6

A pressure-resistant autoclave having a nitrogen gas-blowing inlet wascharged with 90 parts of deionized water, 60 parts of styrene, 1 part oft-dodecyl mercaptan and 4 parts of polyvinyl alcohol (PVA-205, suppliedby Kuraray Co., degree of polymerization: 350, degree of saponification:88.0 mol %). The content was flushed with nitrogen, and 40 parts ofbutadiene was pressed into the autoclave through a pressure-resistantmeter, and the resulting mixture was stirred to give an emulsion ofmonomers.

A pressure-resistant autoclave equipped with a nitrogen gas-blowinginlet and a thermometer was charged with 57 parts of distilled water and3 parts of ethanol, and the mixture was flushed with nitrogen and thenheated to 70° C. Then, while the temperature was maintained at 70° C.,an aqueous 2% ammonium persulfate solution was pressed into theautoclave, and immediately the above-mentioned monomer emulsion wasadded over a period of 5 hours. After the completion of addition,stirring was further continued for 3 hours. Then the conversion wasdetermined. The conversion was 95% (see Table 2). The polymerizationmixture was cooled to give a styrene-butadiene copolymer emulsion F. Thepreparation conditions employed for emulsion F are summarized in Table2. Emulsion F had a solid content of 49.0% and a viscosity of 90 mpa.s.The characteristics of emulsion F were evaluated by the same proceduresas in Example 1 and the results are shown in Table 4.

Comparative Example 1

In this example, polyvinyl alcohol was added only before the addition ofpolymerization initiator and was not added after the commencement ofpolymerization.

To 90 parts of deionized water, 80 parts of styrene and 20 parts ofethyl acrylate was added, and the mixture was stirred to give a monomermixture.

A glass reaction vessel equipped with a reflux condenser, a droppingfunnel, a thermometer, a nitrogen gas-blowing inlet and a stirrer wascharged with 57 parts of distilled water, 3 parts of ethanol and 5 partsof polyvinyl alcohol (PVA-205, supplied by Kuraray Co., degree ofpolymerization: 550, degree of saponification: 88.5 mol %), and themixture was heated to 80° C. Then, while the temperature was maintainedat 80° C., a solution of 0.5 part of ammonium persulfate in 10 parts ofdeionized water was added to the content in the reaction vessel.. Abouttwo minutes after the addition of the polymerization initiator, additionof the above-mentioned monomer mixture into the reaction vessel wascommenced. When one hour elapsed from the commencement of the monomermixture, a coagulated mass occurred, and, when two hours elapsed, thepolymerization system gelled. The test results are shown in Table 2 andTable 4.

Comparative Example 2

In this example, polymerization was carried out by using a surfaceactive agent.

To 90 parts of deionized water, a monomer mixture of 80 parts of styreneand 20 parts of ethyl acrylate, and 5 parts of sodium lauryl sulfatewere added, and the mixture was stirred to give a monomer emulsion.

A glass reaction vessel equipped with a reflux condenser, a droppingfunnel, a thermometer, a nitrogen gas-blowing inlet and a stirrer wascharged with 57 parts of distilled water and 3 parts of ethanol, and themixture was heated to 80° C. Then, while the temperature was maintainedat 80° C., a solution of 0.5 part of ammonium persulfate in 10 parts ofdeionized water was added to the content in the reaction vessel. Abouttwo minutes after the addition of the polymerization initiator, additionof the above-mentioned monomer emulsion into the reaction vessel wascommenced. The addition of the monomer emulsion was continued over aperiod of 4 hours. After the completion of addition, stirring wasfurther continued for 2 hours. Then the conversion was determined andthe polymerization mixture was cooled to give an aqueous polymeremulsion P.

The solid content in emulsion P was adjusted to 35%. The 35% emulsion Phad a pH value of 2.3, a B-type viscosity of 400 mpa.s and a surfacetension of 38 mN/m. The Marlon mechanical stability was 0.0067 at a pHof 8.0 and the calcium chloride chemical stability index was at least 5and smaller than 1. The preparation conditions employed for emulsion Pand the characteristics of emulsion F are shown in Table 2 and Table 4,respectively.

Comparative Example 3

In this example, polymerization was carried out by using a surfaceactive agent and polyvinyl alcohol was added after polymerization.

To emulsion P prepared in Comparative Example 2, 5 parts of polyvinylalcohol (PVA-205, supplied by Kuraray Co., degree of polymerization:550, degree of saponification: 88.5 mol %) was added to give an aqueouspolymer emulsion Q.

The solid content in emulsion Q was adjusted to 35%. The 35% emulsion Phad a pH value of 2.3, a B-type viscosity of 560 mpa.s and a surfacetension of 40 mN/m. The Marlon mechanical stability was 0.0067 at a pHof 8.0. The preparation conditions employed for emulsion Q and thecharacteristics of emulsion Q are shown in Table 2 and Table 4,respectively.

Comparative Example 4

In this example, polymerization was carried out by using PVA as aprotective colloid.

A glass reaction vessel equipped with a ref lux condenser, a droppingfunnel, a thermometer, a nitrogen gas-blowing inlet and a stirrer wascharged with 147 parts of distilled water, 3 parts of ethanol and 5parts of polyvinyl alcohol (PVA-205, supplied by Kuraray Co., degree ofpolymerization: 550, degree of saponification: 88.5 mol %), and then, 80parts of styrene and 20 parts of ethyl acrylate were added. Then themixture was heated to 80° C., and, while the temperature was maintainedat 80° C., a solution of 0.5 part of ammonium persulfate as apolymerization initiator in 10 parts of deionized water was added tocommence the polymerization. When 30 minutes elapsed from thecommencement of polymerization, a coagulated mass occurred, and, whentwo hours elapsed, the polymerization system gelled. The test resultsare shown in Table 2 and Table 4.

Comparative Example 5

In this example, polymerization was carried out in the absence of analcohol.

By the same procedures as described in Example 1, polymerization wascarried out except that the amount of polyvinyl alcohol (PVA-205) wasvaried as shown in Table 2 and 3 parts of water was used instead of 3parts of ethanol, but, the polymerization system gelled. The testresults are shown Table 2 and Table 4.

Comparative Example 6

In this example, polymerization was carried out by usingazobiisobutyronitrile as a polymerization initiator.

By the same procedures as described in Example 1, lymerization wascarried out except that azobiisobutyronitrile (AIBN) was used instead ofammonium persulfate, but, the polymerization system gelled. The testresults are shown in Table 2 and Table 4.

TABLE 1 Example No. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Initial charge:Deionized water 57 57 57 57 45 57 Ethanol 3 3 3 3 15 3 Styrene — — — — —— Ethyl acrylate — — — — — — PVA-205 — — — — — — Polymerizationinitiator: Potassium persulfate — 0.5 0.5 0.5 — — Ammonium persulfate0.5 — — — 0.5 0.2 Deionized water 10 10 10 10 10 10 Monomer emulsion:Deionized water 90 90 90 90 90 90 Styrene 80 40 40 80 80 60 Ethylacrylate 20 — — 20 20 — Butyl acrylate — 60 60 — — — Butadiene — — — — —40 Methacrylic acid — — 2 — 1 — t-Dodecyl mercaptan — — — — — 1 PVA-2055 — — 10 10 4 PVA-220E — 10 10 — — — Sodium lauryl sulfate — — — — — —Polymerization conditions: Polymerization temperature 80 80 80 80 80 70(° C.) Period of time for monomer 4 4 4 4 4 5 addition (hours) PVA-205after-addition — — — — — — Copolymer emulsion A B C D G F

TABLE 2 C.E. C.E. C.E. C.E. C.E. C.E. Comparative Example No. 1 2 3 4 56 Initial charge: Deionized water 57 57 57 147 60 57 Ethanol 3 3 3 3 0 3Styrene — — — 80 — — Ethyl acrylate — — — 20 — — PVA-205 5 — — 5 — —Polymerization initiator: Potassium persulfate — — 0.5 0.5 — — Ammoniumpersulfate 0.5 0.5 — — 0.5 — Deionized water 10 10 10 10 10 10Azobisisobutyronitrile (AIBN) — — — — — 0.5 Monomer emulsion: Deionizedwater 90 90 90 — 90 90 Styrene 80 80 40 — 80 80 Ethyl acrylate 20 20 — —20 20 Butyl acrylate — — 60 — — — Butadiene — — — — — — Methacrylic acid— — — — — — t-Dodecyl mercaptan — — — — — — PVA-205 — — — — 10 10PVA-220E — — — — — — Sodium lauryl sulfate — 5 5 — — — Polymerizationconditions: Polymerization temperature 80 80 80 80 80 80 (° C.) Periodof time for monomer 4 4 4 4 4 4 addition (hours) PVA-205 after-addition— — 5 — — — Copolymer emulsion — P O — — —

TABLE 3 Example No. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Aqueous A B C DG F emulsion Solid content 35 35 35 35 35 35 (%) pH 2.2 2.2 2.2 2.2 2.22.2 Viscosity 1,000 1,200 1,350 1,300 800 900 (mPa · s) Surface tension56 55 56 57 53 50 (mN/m) Average 0.3 0.2 0.2 0.2 0.2 0.3 particlediameter (μm) Particle 0.2 0.1 0.1 0.1 0.1 0.2 diameter ˜0.4 ˜0.3 ˜0.3˜0.3 ˜0.3 ˜0.4 distribution (μm) Marlon 0.0015 0.0018 0.0013 0.0020.0015 0.044 stability (CG8) (%) Calcium >30 >30 >30 >30 >30 >30chloride chemical stability index Graft bond Found Found Found FoundFound Found

TABLE 4 C.E. C.E. C.E. C.E. C.E. C.E. Comparative Example No. 1 2 3 4 56 Aqueous emulsion — P Q — — — Solid content (%) *1 35 35 *1 *1 *1 pH —2.3 2.2 — — — Viscosity (mPa · s) — 500 900 — — — Surface tension (mN/m)— 38 50 — — — Average particle diameter — 0.2 0.2 — — — (μm) Particlediameter — 0.1 0.1 — — — distribution (μm) ˜0.3 ˜0.3 Marlon stability(CG8) (%) — 0.067 0.067 — — — Calcium chloride chemical ≧0.5 ≧20stability index — <0.5 <20 — — — Graft bond — — Not — — — found *1Gelled during polymerization

INDUSTRIAL APPLICABILITY

The polymer emulsion of the invention which is dispersion-stabilizedwith a water-soluble high-molecular-weight compound having an alcoholichydroxyl group has excellent mechanical stability, chemical stability,and water resistance and alcohol dispersibility.

According to the process of the invention wherein an emulsionpolymerization is carried out by using a water-solublehigh-molecular-weight compound having an alcoholic hydroxyl group as adispersing agent, monomers exhibiting a relatively low free-radicalreactivity such as acrylic acid ester monomers, methacrylic acid estermonomers, styrenic monomers and diene monomers, as well as vinyl estermonomers, can be stably polymerized, in a striking contrast to theconventional process by which the monomers having a relatively lowfree-radical reactivity are difficult to polymerize.

In view of the above-mentioned excellent properties, the aqueous polymeremulsion of the invention are useful for an adhesive, a coatingcompound, a processing agent for paper, a binder for inorganicmaterials, an additive for cement and a mortar primer, and it isexpected to be of an enhanced practical use in industry.

We claim:
 1. An aqueous emulsion of a homopolymer consisting of units ofa monomer selected from the group consisting of an acrylic acid estermonomer, a methacrylic acid ester monomer, a styrenic monomer and adiene monomer, or a copolymer predominantly comprised of units of atleast one monomer selected from said group of monomers, which isdispersion-stabilized with a vinyl alcohol polymer having a molecularweight of at least 300 and having an alcoholic hydroxyl group, aviscosity average degree of polymerization of 50 to 8,000, and a degreeof saponification of 40% to 99.99%; characterized in that saidhomopolymer or copolymer is in the form of particles having an averageparticle diameter of 0.05 μm to 5 μm and said aqueous emulsion has acalcium chloride chemical stability index of at least 20 and a surfacetension of at least 45 mN/m as measured at a solid content concentrationof 30% by weight; that the amount of the vinyl alcohol polymer is in therange of 0.01 to 100 parts by weight based on 100 parts by weight of thehomopolymer or the copolymer; that at least part of the vinyl alcoholpolymer is graft-bonded to the homopolymer or copolymer; and furtherthat the amount of the homopolymer or copolymer having graft-bondedthereto the vinyl alcohol polymer is in the range of 0.5% to 30% byweight based on the sum of the homopolymer or copolymer having graftedthereto the vinyl alcohol polymer and the homopolymer or copolymer, towhich the vinyl alcohol polymer has not been grafted.
 2. A process forproducing the aqueous emulsion as claimed in claim 1, characterized inthat either one kind of a monomer selected from the group consisting ofan acrylic acid ester monomer, a methacrylic acid ester monomer, astyrenic monomer and a diene monomer, or a monomer mixture predominantlycomprised of at least one kind of a monomer selected from said group ofmonomers, is polymerized in an aqueous medium in the presence of analcohol and a polymerization initiator generating a peroxidefree-radical as the sole polymerization initiator, while the monomer ormonomer mixture and a vinyl alcohol polymer having a molecular weight ofat least 300 and having an alcoholic hydroxyl group, a viscosity averagedegree of polymerization of 50 to 8,000, and a degree of saponificationof 40% to 99.99%, are incorporated continuously or intermittently into apolymerization system.
 3. The production process according to claim 2,wherein the amount of the vinyl alcohol polymer is in the range of 0.01to 100 parts by weight based on 100 parts by weight of the monomer ormonomer mixture.
 4. The production process according to claim 1, whereinthe amount of the alcohol is in the range of 0.5 to 50 parts by weightbased on 100 parts by weight of the monomer or monomer mixture.
 5. Theproduction process according to claim 1, wherein the polymerizationinitiator is a persulfate salt.
 6. The production process according toclaim 2, wherein the monomer or monomer mixture is incorporated into thepolymerization system at a rate such that the polymerization conversionis maintained at 10% by weight or more during the polymerization.
 7. Theproduction process according to claim 2 wherein the monomer or monomermixture and the vinyl alcohol polymer are simultaneously incorporatedinto the polymerization system.